This invention relates to foamable polyvinylidene fluoride that is useful in forming insulation for electrical wire as well as cable jacketing for such insulated wires.
Usage of polyvinylidene fluoride (PVDF) and copolymers containing a major proportion of PVDF in wire insulation and cable jacketing has grown recently because the relatively low flammability and smoke generation of PVDF permits insulation in plenum duct-work without the need for a costly conduit covering. Accordingly, overall costs and economics of PVDF plenum wiring cable is thus competitive with less expensive insulation materials such as polyvinyl chloride which require conduit shrouding.
The inherently high dielectric constant for PVDF (8-10) renders it unsuitable normally for applications such as wire insulation in long length telephone cables where a low dielectric constant is essential to minimize cross-talk interference. Typical polyvinyl chloride compounds which are currently used have dielectric constants in the 3 to 3.5 range. Through the practice of the instant invention the dielectric constant of PVDF can be reduced to a comparable level by foaming the PVDF material during the wire coating process. The log of the dielectric constant of the foamed PVDF structure is directly proportional to the volume fraction of PVDF with the remainder of the volume being air spaces provided by the foaming process. A PVDF foam with 55% void content has a dielectric constant of about 3.5 and is therefore suitable for many wire insulation applications, including long length telephone primaries.